How to read a single PV when the engine is running a plan?

[stan-dot]

As a user I'd like to see visual feedback on the screen when a xy stage is moving. This most likely web based presentation of information would reassure me that the plan is going according to plan.

Discussions of this feature to date include possible solutions:

• purely pvws based implementation https://pvws.diamond.ac.uk/pvws/
• lightweight 'ophyd as a service' implementation where RunEngine with a single command can get the data, and return to the web GUI through websockets or graphql

The former requires a second configuration mapping devices to PVs, competing with dodal as a source of truth. That would be a maintenance overhead that is now absent from GDA.

The latter would need to be connected to a control-baton service to ensure no invalid changes to the beamline state happen - for instance a move on y axis when the main RunEngine is running a raster scan.

BUT we need a control-baton service anyway for the SWIFT use case with two end stations. Presumably blueapi would need to get a baton to get control of the beamline to change it at the start of a scan.